[Dy2stat] Add Support for a, b = static_variable Grammar (#33499)

For python, if users write `a, b = var`, the `__getitem__` method will iterate through 0, 1, 2 ... until `__getitem__` throws an IndexError, then stop. The var[0], var[1] will be given to a, b respectively. If more values are given, the unpack size would cause error.

We didn't raise the IndexError in the past and we add statement in `__getitem__` to raise IndexError here to support grammar like `a, b = var` in this PR.

python/paddle/fluid/tests/unittests/dygraph_to_static/test_tensor_shape.py

@@ -85,6 +85,13 @@ def dyfunc_tuple_shape_2(x):
     return res
 
 
+def dyfunc_tuple_shape_3(x):
+    x = paddle.to_tensor(x)
+    a, b = paddle.shape(x)
+    res = paddle.reshape(x, shape=(b, a))
+    return res
+
+
 def dyfunc_paddle_shape_api(x):
     x = paddle.to_tensor(x)
     # paddle.shape will not be converted.
@@ -337,6 +344,18 @@ class TestTupleShape2(TestTensorShapeBasic):
         self.expected_slice_op_num = 2
 
 
+class TestTupleShape3(TestTensorShapeBasic):
+    def init_test_func(self):
+        self.input = numpy.ones((5, 7)).astype("int32")
+        self.input_spec = [paddle.static.InputSpec(shape=[5, 7], dtype="int32")]
+        self.dygraph_func = dyfunc_tuple_shape_3
+
+    def _set_expected_op_num(self):
+        self.expected_op_num = 5
+        self.expected_shape_op_num = 1
+        self.expected_slice_op_num = 2
+
+
 class TestPaddleShapeApi(TestTensorShapeBasic):
     def init_test_func(self):
         self.input = numpy.ones((5, 7)).astype("int32")

python/paddle/fluid/variable_index.py

@@ -116,6 +116,19 @@ def _getitem_impl_(var, item):
 
     for dim, slice_item in enumerate(item):
         if is_integer_or_scalar_tensor(slice_item):
+            if isinstance(slice_item,
+                          int) and var.shape[dim] is not None and var.shape[
+                              dim] >= 0 and slice_item >= var.shape[dim]:
+                # For python, if users write a, b = var, the __getitem__
+                # method will iterate through 0, 1, 2 ... until __getitem__
+                # throws an IndexError, then stop. The var[0], var[1] will
+                # be given to a, b respectively. If more values are given,
+                # the unpack size would cause error.
+                #
+                # We raises IndexError here to support grammar like `a, b = var`
+                raise IndexError(
+                    "slice_item %d at dim %d should be >= 0 and < var.shape[%d]: %d"
+                    % (slice_item, dim, dim, var.shape[dim]))
             decrease_axes.append(dim)
             start = slice_item
             step = 1